Apparatus for producing a beam of accelerated liquid metal droplets



Sept.

LIQUID METAL DROPLETS S. SOWA APPARATUS FOR PRODUCING A BEAM OF ACCELERATED Filed April 6, 1965 W V 'I'l g 6 1 as 2.2 I

42 Jb 5 in INVENTOR Edmurid 5. Souaa United States Patent T 3,341,720 APPARATUS FOR PRODUCING A BEAM CF AC- CELERATED LIQUID METAL DROIPLETS Edmund S. Sowa, Westmount, Ill., assignor to the United States of America as represented by the United States Atomic Energy Commission Filed Apr. 6, 1965, Ser. No. 446,136 6 Claims. (Cl. 310-) ABSTRACT OF THE DISCLOSURE An apparatus for producing a beam of accelerated liquid metal droplets includes liquid metal fed to a nozzle at whose exit orifice liquid metal droplets are formed. The liquid metal is maintained at electrical ground. A source of alternating voltage is provided having output terminals in phase opposition and symmetrical with respect to electrical ground. A plurality of accelerating electrodes fixed in increasing spatial relationship are mounted along a line which includes the exit orifice of the nozzle. The first of the electrodes is connected to one output terminal of the voltage source and is located adjacent the exit orifice of the nozzle to induce a charge in the liquid metal droplets as they are formed and accelerate the charged droplets away from the exit orifice of the nozzle toward the first electrode. The remaining electrodes are successively connected to alternate terminals of the voltage source and the spacing between the electrodes is related to the frequency and amplitude of the voltage source and to the charge-to-mass ratio of the liquid metal droplets so as to cause the droplets to experience a unidirectional accelerating force.

This invention relates to apparatus and method for producing a beam of alternately charged groups of accelerated liquid metal droplets, and more particularly, to apparatus and method for producing alternately charged groups of accelerated liquid metal droplets using an oscillating electric field to induce a charge in the droplets as they form and accelerate them through the field as determined by the spacing and geometry of fixed electrodes.

The production of charged particulate matter has been the subject of many recent research efforts, see generally, Quest for Heavy Particle Electric Propulsion, R. E. Hunter, Advanced Propulsion Concepts, Gordon and Breach, 1963, volume I, page 19, and references following the article. Present approaches to this problem leave many obstacles to be overcome. Where a DC electric field is used to accelerate charged particulate matter, a net charge accumulation results in the accelerant with respect to the accelerator. Any scheme to neutralize this voltage differential results in the addition of hardware to the accelerator and reduction in the overall efiiciency of the machine, Electric Propulsion, Ernst Stuhlinger, Electrical Engineering, volume 82, 1963, page 459. The present invention overcomes this difficulty by producing groups of charged liquid metal droplets in the form of a pulsed beam wherein the charge of each successive group is of reversed polarity, resulting in negligible net charge accumulation in the beam.

Another difficulty in the use of non-time-varying fields to accelerate charged particles is the production of high intensity DC fields. The use of an alternating field obviates the requirement for rectification when an AC source is used to supply power.

Most present day schemes employ ions or a plasma as the accelerant, see Advanced Propulsion Concepts, supra. The development of ion or plasma sources presents diflicult engineering problems since these sources usually 3,341,720 Patented Sept. 12, 1967 operate under extreme temperature stresses or require adequate shielding where nuclear radiation is used to produce the charged mass. Further, systems designed to accelerate ions require the electric field to accelerate the ions to a higher velocity than is required for heavier particles in order to develop the same thrust, due to the relatively small mass of the ions. The use of droplets, having a larger mass than ions, significantly reduces the velocity to which the charged particles must be accelerated to produce a given thrust.

Once the droplets have been accelerated to a sufiicient velocity, the kinetic energy of the droplets may be used for various purposes. One such purpose is to develop a very high DC potential by dividing the beam of alternately charged group of droplets into separate homogeneously charged beams and collecting the separated beams in electrically isolated containers. Separation is accomplished by passing the original beam through an electrostatic or magnetic field. The kinetic energy of the droplets is used to overcome the repulsive force exerted by the similarly charged containers. As the droplets are collected in the containers, charge of opposite polarity accumulates in the respective containers, and a substantial voltage differential builds up between the two containers.

It is, therefore, an object of this invention to produce uid metal droplets.

It is another object of this invention to produce a beam of alternately charged groups of accelerated liquid metal droplets using an oscillating voltage as the source of ex citation.

It is a further object of this invention to produce a beam of alternately charged groups of accelerated liquid metal droplets wherein the net charge accumulation of the accelerant is negligible.

It is yet another object of this invention to produce a beam of alternately charged groups of accelerated liquid metal droplets wherein both the source of charged droplets and the accelerating chamber operate at ambient temperatures.

It is an even further object of this invention to generate a very high DC potential from a beam of alternately charged groups of accelerated liquid metal droplets.

Briefly, the objects of the present invention are accomplished as described in this paragraph. Accelerating electrodes are mounted along a line with an increasing interval between electrodes as one proceeds from the first to the last. Alternate electrodes are connected in common and an alternating voltage power source is used to energize the electrodes by connecting one polarity of the source to one commonly grouped set of electrodes and the other polarity of the power source to the second set of commonly grouped electrodes. In operation, alternate polarity electric fields travel along the line determined by the electrodes. The field is similar to the electric fields produced in linear accelerators, see generally, Particle Accelerators, Livingston and Bleweet, McGraw-Hill (1962) chapter 10. Liquid metal droplets of uniform diameter are formed at a nozzle adjacent to the first electrode. As the electrode is energized by the power source, it induces a charge of opposite polarity in the droplets being formed. The droplets are then accelerated toward the inducing electrode and thence by the traveling electric field. As the first group of droplets passes the first electrode, the polarity of the source changes, reversing the field between electrodes. This group is then attracted to the second electrode while the first electrode acts both to repel this group and to induce a charge of opposite polarity in a second group of droplets forming at the nozzle. The second group of droplets is then accelerated toward the first reversing polarity.

This beam of alternately charged groups of accelerated liquid metal droplets may be used in a variety of ways. For example, in order to generate a very high DC potential, an electrostatic field is formed perpendicular to the line of travel of the beam to separate droplets of positive charge from droplets of negative charge. The positively charged droplets are then allowed to collect in one container and the negatively charged droplets to collect in a second container which is electrically isolated from the first container. A DC potential thus builds up between these two containers as charge accumulates, limited only by the kinetic energy of the accelerated droplets.

The present invention may also be put to use as a propulsion device or thrust producer. Obviously there is a force on the accelerating apparatus as a whole which is equal in magnitude and opposite in direction to the force exerted thereby on the droplets. This equal and opposite force may be used to accelerate the accelerating apparatus as a Whole relative to the droplets, similar to the ion and plasma engines described in Advanced Propulsion Concepts, supra. 7

Further understanding of the present invention may better be obtained by consideration of the accompanying drawing which is a schematic diagram of apparatus for the practice of the invention.

Referring to the drawing, electrodes 8, 9, 10 and 11 are constructed of metal tubing bent in a circle forming a to'rroid. The electrodes 8, 9, 10 and 11 are held in place by electrode positioning brackets 12 and 13. The metal brackets 12 and 13 are mounted on electrode supporting rods 14 and 15, respectively, which are also metal. The brackets 12 and 13 are adjusted so that the centers of the torroidal electrodes 8, 9, 10 and 11 lie in a straight line which is perpendicular to the plate of the torroid. The electrodes 8, 9, 10 and 11 are spaced at increasing intervals for reasons and in a manner explained below.

The straight line passing through the centers of the torroidal electrodes 8, 9, 10 and 11 includes an orifice 16 of a nozzle 18 at which liquid metal droplets are formed The liquid metal, which in the present device is the eutectic alloy of sodium and potassium'commonly known as NaK, is stored in container 30. The NaK is fed into the nozzle 18 at an opening 28 through flexible tubing 32.

Mechanically coupled to the'nozzle 18 is a transducer 33 for converting ultrasonic electrical power supplied at excitation terminals 36 of the transducer 33 into mechanical vibrations. Ultrasonic vibration thus supplied to the nozzle 18 helps to produce droplets of more uniform diameter at orifice 16.

The electrical energy needed to induce a charge and accelerate the droplets is supplied at terminals 22 of step-up transformer 20. The secondary winding of transformer has a center tap 25 which is grounded. The output terminals 24 of transformer 20 are connected through current-limiting resistors 26 to the electrode supporting rods 14 and 15, respectively. The nozzle 18 is also grounded, keeping it and the mass of NaKrat a constant potential.

When the excitation voltage is supplied to terminals 22 of transformer 20, the electrode 8 adjacent to the orifice 16 induces a charge in the liquid metal droplets forming at the orifice 16. The polarity of this induced charge is the reverse of the polarity on the inducing electrode 8.

Assuming that electrode support rod 14 is positive as excited by transformer 20, the electrodes 8 and 10 are also positive with respect to ground. At the same time electrodes 9 and 11 are negative with respect to ground. The electrode 8 which is nearest the orifice 16 acts during droplet formation to repel positive charges from and to attract electrons to the orifice 16. While electrode 8 is positive, droplets are formed with a net negative charge. This charge then acts to repel the formed droplet from the negatively-charged orifice 16 and to attract the droplet toward electrode 8 by the force of attraction of opposite charges. A number of such droplets will be formed during one-half cycle of the alternating voltage supplied at the input terminals 22. These negativelycharged droplets will be attracted as a group to the electrode 8.

As the excitation voltage supplied to transformer 20 changes phase, the field established by the electrodes 8,

9, 1t) and 11 collapses and the momentum of this first group of negatively charged droplets carries the group through the opening of torroidal electrode 8. The electrode 8 then assumes negative potential repelling this first group of'charged droplets, and at the same time, electrode 8 induces a positive charge in droplets now forming at the orifice 16. Electrode 9 is positive at this time and acts to attract the first group of droplets traveling between'electrode 8 and electrode 9. As the droplets are accelerated between electrodes there is an increase in the velocity of the droplets. The spacing of the electrodes 8, 9, 10 and 11 is such that the electrical fieldgenerated thereby exerts a unidirectional accelerating force on the charged droplets in flight between electrodes. The field is negligible as the polarity of the electrodes 8, 9, 10 and 11 changes and the droplets pass through the center of the electrodes 8, 9, 10 and 11. The effect is a unidirectional force on the droplets by simulating an electric field traveling from the orifice 16 along the electrodes 8, 9, 10 and 11 at the same velocity as the droplets, with the increased spacing between electrodes 8, 9, 10 and 11 along the flight path compensating for the increase in velocity of the droplets as they are accelerated.

The electric field generated by electrodes 8, 9, 10 and 11 is similar to the traveling field used in linear accelerators as mentioned above. The field when excited by an AC source, however, is periodically oscillatory with respect to time at any one point between electrodes. The space between electrodes 8, 9, 10 and 11 is related to the frequency and amplitude of the excitation voltage supplied to transformer 20 and the charge-to-mass ratio of the liquid metal droplets so as to produce the unidirectional force on the droplets.

The orifice 16 at which the droplets are formed, the electrodes 8, 9, 10 and 11, the electrode positioning brackets 12 and 13, and the lower portion'of electrode supporting rods 14 and 15 are enclosed in a bell jar 39 evacuated by a vacuum pump 38. It has been found that a pressure of 10- to l0 torr is suitable for success ful operation of the apparatus. The purpose of the vacuum is to prevent arc discharging between electrodes and between droplets and electrodes. Current limiting resistors 26 are placed in series with the output terminals 24 of transformer 20 in order to minimize current flow if arcing does occur.

In the embodiment illustrated in the drawing, satisfactory results have been obtained with the following parameters being fixed as indicated: the center of the torroid of the first electrode 8 is placed 0.5 inch directly below the orifice 16; the distance between electrode 8 and electrode 9 is 1.7 inches; the distance between electrode 9 and electrode 10 is 3.0 inches; the distance between electrode 10 and electrode 11 is 4.5 inches. For this par ticular experiment, acceleration was measured by high speed photographic methods. The camera viewed only the electrode 8, the electrode 9, and the orifice 16.

The nozzle 18 is a hypodermic needle one-half inch long with inside diameter of 0.011 inch and having a square end at orifice 16. The secondary voltage of trans,-

former 20 has an RMS voltage of 3,000 volts with respect to ground, which results in a maximum peak-to-peak voltage of 8,460 volts between electrodes. The frequency of the voltage supplied to transformer 20 was fifteen cycles per second for the above-mentioned electrode spacing and applied voltage. Droplet sizes were of the order of onehalf millimeter in diameter. The ultrasonic excitation supplied by transducer 33 was set at a frequency of twenty thousand cycles per second. For these values of system parameters, groups of droplets in numbers of five to ten per half cycle were accelerated to a velocity which, measured at the second electrode 9 was approximately 19.4 feet per second. From this, it is estimated that the terminal velocity was approximately 60 ft./sec.

Compensating for an initial velocity due to the force created by the pressure differential existing between the source of NaK which was at atmospheric pressure and the partial vacuum within the bell jar, the effective acceleration on the droplets under the combined influence of the electric field and gravity traveling from the nozzle 16 to the electrode 9 was 1850 ft./sec.

When used as a thrust producer, for example as a propulsion engine for a rocket in space, the force exerted on the droplets would also be experienced on the apparatus as a whole in the opposite direction. On the basis of the above results, the thrust per unit time is approximately 8X10- pounds per droplet per second, and each droplet would have a terminal kinetic energy of 7.0)(10 ft.-lb. In this space application, the bell jar could, of course, be opened up at the bottom or simply eliminated. As mentioned before, there would be no need to neutralize the charge on the droplets as they were expelled from the apparatus since the succeeding group of droplets would be of opposite charge and neutralize the charge on the first group. This i an advantage over ion engines wherein the propellant is of one polarity charge. By accelerating the droplets through a longer distance, a larger force could, of course, be generated reducing the force of gravity to a negligible amount and allowing operation of the apparatus in arbitrary positions with respect to the vertical.

In another application of this invention, this beam may be separated into two beams of homogeneous charge by establishing an electrical field perpendicular to the direction of travel of the beam and intersecting the beam. For this purpose deflecting plates 40 and 41 are provided. Plate 40 is biased by a battery 42 as a potential positive with respect to ground; plate 41 is biased by a battery 43 at a potential negative with respect to ground. Emerging from the field between plates 40 and 41 will be two divergent beams of homogeneously charged liquid metal droplets. The beam containing positively charged droplets is collected in a container or collector 44. Likewise, the beam containing negatively charged droplets is collected in a container or collector 46. Feed-through leads 48 connect high voltage output terminals 50 through the bell jar to the collectors 44 and 46. Voltage is built up between the terminals 50 limited only by the kinetic energy of the droplets used to overcome the repulsive force of the respective collectors 44 and 46.

It is to be further noted that many equivalents exist which produce the simulated traveling electric field described above. Once such system is to remove the ground connectors of the center tap 25 of transformer 20 and connect one secondary terminal of transformer 20 to electrode supporting rod 14. The other secondary terminal of transformer 20 would then be connected to electrode supporting rod and also to the nozzle 18. Other equivalents to producing the traveling field include a multiphase power source with a common terminal connected to the nozzle and grounded and with successive phases connected to successive electrodes 8, 9, 10 and 11. The spacing of electrodes may, of course, change for such an arrangement. The number of electrodes is obviously not limited to the number mentioned.

Persons skilled in the art will readily adapt the teach- .ings of the present invention to uses far different from those illustrated. Accordingly, the scope of protection afforded the invention should be determined only in accordance with the appended claims and not limited to the uses above described.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. Apparatus for producing alternately charged groups of accelerated liquid metal droplets comprising: a contained mass of liquid metal; means for forming droplets of substantially uniform diameter at a location on a surface of said liquid metal; an alternating voltage power source having a first output terminal of one polarity connected to said liquid metal; and a plurality of accelerating electrodes fixed in increasing spatial relationship along a line which includes the location of droplet formation, the first of said electrodes being connected to an output terminal of opposite polarity than said first output terminal and being located adjacent the location of droplet formation to induce a charge in said droplets as they are formed and to accelerate said droplets away from said location of formation and toward said first electrode, the remaining electrodes being successively connected to alternate polarities of said power source, the spacing between successive electrode being related to the frequency and amplitude of said power source and the charge-tomass ratio of said droplets so as to cause said droplets to experience a unidirectional accelerating force.

2. The apparatus of claim 1 wherein the droplet formation means comprises at least one elongated tubular duct having an orifice, said liquid metal passing through said duct to form into droplet at said orifice.

3. The apparatus of claim 2 further including ultrasonic transducer means coupled to said droplet formation means to induce ultrasonic vibrations therein.

4. The apparatus of claim 1 wherein the accelerating electrodes are formed to enclose a substantially planar area, the flow of charged droplets being perpendicular to the plane of the electrode and within the area enclosed thereby.

5. Apparatus for producing alternately charged groups of accelerated liquid metal droplet comprising: a contained mass of liquid metal at ground potential; means for forming droplets of substantially uniform diameter at a location on a surface of said liquid metal; a source of alternating voltage power having two output terminals in phase opposition and symmetrical with respect to ground potential; and a plurality of accelerating electrodes fixed in increasing spatial relationship along a line which includes the location of droplet formation, the first of said electrodes being connected to one output terminal of said power source and being located adjacent the location of droplet formation to induce a charge in said droplets as they are formed and to accelerate said charged droplets away from said location of formation and toward said first electrode, the remaining electrodes being successively connected to alternate terminals of said power source, the spacing between successive electrodes being related to the frequency and amplitude of said power source and to the charge-to-mass ratio of said droplets so as to cause said droplets to experience a unidirectional accelerating force.

6. Apparatus for converting low voltage AC energy to high voltage DC energy comprising: a contained mass of liquid metal; means for forming droplets of substantially uniform diameter at a location on a surface of said liquid metal; an alternating voltage power source having a first output terminal of one polarity connected to said liquid metal; a plurality of accelerating electrodes fixed in increasing spatial relationship along a line which includes the location of droplet formation, the first of said electrodes being connected to an output terminal of opposite polarity than said first output terminal and located adjacent the location of droplet formation to induce a charge in said droplets as they are formed and to accelerate said droplets away from said location of formation and toward said first electrode, the remaining electrodes being successively connected to alternate polarities of said power source, the spacing between successive electrodes being related to the frequency and amplitude of said power source and the charge-to-mass ratio of said droplets so as to cause said droplets to experience a unidirectional accelerating force; means for establishing an electrostatic DC field substantially perpendicular to and intersecting the alternately charged beam of liquid metal droplets emanating from the last of said accelerating electrodes, whereby two divergent beams of homogeneous charge emerge from said electrostatic field; a first container means placed tocollect the charged droplets composing one of said divergent beams; and a second container means, electrically isolated from said first container, placed to collect the charged droplets composing the other divergent 5 beam.

References Cited UNITED STATES PATENTS 3,191,077 6/1955 Marks et a1. 313-63 X 10 2,896,083 7/1959 Hare et a1. 313-63 X JOHN F. COUCH, Primary Examiner.

W. H. BEHA, IR.,'Assistant Examiner. 

1. APPARATUS FOR PRODUCING ALTERNATELY CHARGED GROUPS OF ACCELERATED LIQUID METAL DROPLETS COMPRISING: A CONTAINED MASS OF LIQUID METAL; MEANS FOR FORMING DROPLETS OF SUBSTANTIALLY UNIFORM DIAMETER AT A LOCATION ON A SURFACE OF SAID LIQUID METAL; AN ALTERNATING VOLTAGE POWER SOURCE HAVING A FIRST OUTPUT TERMINAL OF ONE POLARITY CONNECTED TO SAID LIQUID METAL; AND A PLURALITY OF ACCELERATING ELECTRODES FIXED IN INCREASING SPATIAL RELATIONSHIP ALONG A LINE WHICH INCLUDES THE LOCATION OF DROPLET FORMATION, THE FIRST OF SAID ELECTRODES BEING CONNECTED TO AN OUTPUT TERMINAL OF OPPOSITE POLARITY THAN SAID FIRST OUTPUT TERMINAL AND BEING LOCATED ADJACENT THE LOCATION OF DROPLET FORMATION TO INDUCE A CHARGE IN SAID DROPLETS AS THEY ARE FORMED AND TO ACCELERATE SAID DROPLETS AWAY FROM SAID LOCATION OF FORMATION AND TOWAQRD SAID FIRST ELECTRODE, THE 